As a contractor, I have been in this situation a few times. As stated above, you need to pay for the contractor to give you this information since the entire project is speculatory on IF you purchase the property
What I have done in the past is charged a $250 1 hour walkthrough with the customer. At that time I point out any major problems that I see as being potential costs and listen to what the customer wants.
Based on that information I give them general costs in an estimate. If the customer ends up buying the house I will credit the $250 paid towards the overall project & once hired will release an itemized breakdown of project expenses.
Estimating rehab costs is no small task. This is not something that can easily be done in an hour. Renovation budgets can take days to prepare in some cases and you should always have a contingency budget. It seems the cost is always greater as you encounter undiscovered problems, which will happen.
What I would recommend doing is figuring out what the after renovation value will be and create your own realistic budget for rehab costs. By doing this you may be able to self predict if the amount of work that needs to be done even seems remotely realistic prior to involving a contractor.